Question: 5 people can paint 3 walls in 49 minutes. How many minutes will it take for 9 people to paint 10 walls? Round to the nearest minute.
Solution: We know the following about the number of walls $w$ painted by $p$ people in $t$ minutes at a constant rate $r$ $w = r \cdot t \cdot p$ $\begin{align*}w &= 3\text{ walls}\\ p &= 5\text{ people}\\ t &= 49\text{ minutes}\end{align*}$ Substituting known values and solving for $r$ $r = \dfrac{w}{t \cdot p}= \dfrac{3}{49 \cdot 5} = \dfrac{3}{245}\text{ walls painted per minute per person}$ We can now calculate the amount of time to paint 10 walls with 9 people. $t = \dfrac{w}{r \cdot p} = \dfrac{10}{\dfrac{3}{245} \cdot 9} = \dfrac{10}{\dfrac{27}{245}} = \dfrac{2450}{27}\text{ minutes}$ $= 90 \dfrac{20}{27}\text{ minutes}$ Round to the nearest minute: $t = 91\text{ minutes}$